Also, what about the proximity effect due to using litz wire (multiple strands in general) vs using a single conductor? I believe it is a trade off between losses due to skin effect and losses due to proximity effect. Any general guidelines about this?
This is what you are thinking of
one point of view see here http://ep2000.com/uploads/EP_WhitePaper_SkinEffect.pdf
the other point of view given at some where in litzwire.com
Proximity effect is the tendency for current to flow in loops or concentrated distributions due to the presence of magnetic fields generated by nearby conductors. In transformers and inductors, proximity effect losses are generally more significant than skin effect losses. In Litz wire windings, proximity effect may be sub-divided into internal proximity effect (the effect of other currents within the bundle) and outer proximity effect ( the effect of the current in other bundles). The reason for twisting or weaving Litz wire, rather than just grouping fine conductors together, is to ensure that the strand currents are equal
. Simple twisted bunched conductor wire can accomplish this adequately where proximity effect would be the only significant problem with solid wire. Where skin effect would also be a problem, more complex Litz wire constructions can be used to ensure equal strand currents. Therefore, in a well-designed construction, strand currents are nearly equal.
For further detail search through that site.